Many diseases and disorders are caused by inappropriate or excessive activation of signal transduction pathways caused by activation of cell surface receptors, e.g., by the binding of receptor-specific ligands. Receptors involved in the initiation or progression of diseases and disorders, such as cancer and autoimmune disorders, have emerged as prime targets for the development of therapeutics that reduce or prevent receptor activation. Examples of target receptors include, e.g., the epidermal growth factor receptor (“EGFR”), the insulin-like growth factor 1 receptor (“IGF1-R”), and the platelet-derived growth factor receptor (“PDGFR”), which tend to be overexpressed or aberrantly activated in many disease states, such as in the most common solid tumors, including non-small cell lung cancer and cancers of the breast, prostate, and colon, and in many autoimmune disease, such as myasthenia gravis, systemic lupus erythematosus, and rheumatoid arthritis. Activation of the receptor results in autophosphorylation, which drives signal transduction pathways that lead to disease progression.
Seminal studies with receptor inhibitors have clearly demonstrated that by preventing the activation of a receptor associated with a disease state the development of that disease state can be altered. Generally, though, the receptor or receptors responsible for the disease state are expressed on many different cells and tissues in addition to the diseased cells or tissues. Although receptor inhibitors, e.g., Herceptin®, which targets ErbB2 (“HER-2”), are becoming available for clinical use, new challenges include identifying a therapeutic agent that will effectively target the diseased cells or tissue without targeting non-affected cells and tissues.
One approach to targeting agents specifically to diseased cells has been the use of bispecific binding agents, sometimes referred to herein as “bsBAs”. Bispecific binding agents comprise two binding domains, each of which specifically recognizes and binds to a separate molecule (for convenience, the molecule specifically bound by each respective binding domain may be referred to as the “ligand” for that binding domain). Bispecific binding agents have been attempted for some time, as exemplified by Schmidt M, et al., “A bivalent single-chain antibody-toxin specific for ErbB-2 and the EGF receptor,” Int J Cancer, 65(4):538-46 (1996), Lu D, et al., “Simultaneous blockade of both the epidermal growth factor receptor and the insulin-like growth factor receptor signaling pathways in cancer cells with a fully human recombinant bispecific antibody,” J Biol. Chem. 279(4):2856-65 (2004), and Francois C, et al., “Antibodies directed at mouse IL-2-R alpha and beta chains act in synergy to abolish T-cell proliferation in vitro and delayed type hypersensitivity reaction in vivo,” Transpl Int. 9(1):46-50 (1996). Because bsBAs often use antibodies as one or both of the binding domains, bsBAs are sometimes included in the class of agents referred to as immunotherapeutics.
Unfortunately, the universe of molecules that can be used as targets for bsBAs is limited. Only a relatively small number of molecules are expressed on diseased cells but not on normal cells, and which therefore can be used to target agents exclusively to diseased cells. An additional number of molecules are expressed in greater numbers on diseased cells than on normal cells. These molecules can permit some preferential delivery of agents to diseased cells over normal cells, depending on the degree to which the molecule is overexpressed in diseased cells compared to normal cells.
Even with substantial overexpression of the target molecule on target cells, however, delivery of targeted therapeutic agents have often been accompanied by adverse side effects due to binding of the agent to normal cells expressing the target molecule. For example, the HER2 (erbB2) receptor that is the target for the FDA-approved immunotherapeutic agent Herceptin®, is overexpressed at levels some 10 to 100 times more than the expression of the HER2 receptor in non-cancer cells. Nonetheless, a percentage of patients develop cardiac arrhythmia and other adverse side effects due to binding of Herceptin® to normal cells.
Thus, it would be desirable to increase the therapeutic window of immunotherapeutic agents by developing bsBAs with an improved ability to bind to target cells without binding to non-target cells.